Generally a casing is sealed in the wellbore by a layer of cement between the casing and the wellbore wall. However it is known that cement does not always provide the desired degree of sealing in view of shrinkage of the cement during curing. As a result of such shrinkage, micro-annuli can occur in the cement which form a migration path leading to undesired flow of fluid through the wellbore.
Further, it has been proposed to seal an annular space between a tubular element arranged in the wellbore, and a wall extending around the tubular element, such as the wellbore wall, by means of an annular seal made of a material which swells upon contact with hydrocarbon fluid. The annular seal is connected to the outer surface of the tubular element, and the tubular element with the annular seal connected thereto is then lowered into the wellbore. When hydrocarbon fluid flows into the annular space, the seal swells and thereby seals the annular space. Although such method of sealing has provided good results, an inherent problem is the occurrence of possible damage to the seal during lowering of the tubular element into the wellbore, particularly if the annular space is narrow. In some useful applications the annular space is even extremely narrow so that proper lowering of the tubular element with the seal connected thereto is impossible.
Also, adequate sealing will be difficult if the space to be sealed has an irregular shape. For example, if the space is (partly) defined by the wellbore wall, adequate sealing using an annular seal can be impossible if the wellbore has washed-out sections which inadvertently occurred during drilling of the wellbore.